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Basilosaurus cetoides

Philip D. Gingerich, University of Michigan

Basilosaurus cetoides is the official state fossil of Alabama. In life, this species was a large serpentine sea creature that lived at the end of the Eocene epoch of the Cenozoic, approximately 34 to 35 million years ago. Basilosaurus was first thought to be a reptile but was later identified as an ancient type of whale (Order Cetacea, Suborder Archaeoceti) that grew to 60 or 70 feet (18 to 20 meters) long. Most of what scientists know about Basilosaurus is based on fossils found in Alabama.

Basilosaurus cetoidesThe Eocene epoch, when Basilosaurus lived, marked the end of a long warm interval in Earth's history, with a climate carried over from the Mesozoic age of dinosaurs. The shallow sea that covered southern Alabama, precursor to today's Gulf of Mexico, teemed with marine life. Basilosaurus was a predator and undoubtedly fed on the larger fish and other vertebrates in such an environment. Fossils are easily buried and preserved in shallow seas, which explains why Alabama is so rich in marine fossils. Early horses and other modern mammal groups undoubtedly roamed the forested uplands of northern Alabama during the same time period when Basilosaurus lived, but erosion has removed all traces of these creatures.

The first Basilosaurus fossil to come to the attention of scientists was a single vertebra, or back-bone, found in Louisiana in 1832 and sent to the American Philosophical Society in Philadelphia. Natural historian Richard Harlan described the fossil in a brief report, published in 1834, in which he named the animal Basilosaurus, or "king lizard." Harlan received more Basilosaurus material from John G. Creagh, a judge in Clarke County, Alabama, in 1834. Harlan took teeth, jaws, and other bones to England in 1839, where they were examined by Richard Owen, a renowned professor of comparative anatomy at the Royal College of Surgeons. At the time, Owen was trying to determine which giant reptiles belonged in the group he termed Dinosauria. After examining Harlan's specimens, Owen concluded that Basilosaurus was a mammal, not a dinosaur or other reptile, because it had double-rooted teeth. He then proposed that the species be renamed Zeuglodon cetoides, choosing the genus name for its reference to the creature's teeth and the species name in recognition of its relationship to whales. The name Basilosaurus was retained, however.

In the mid-1890s, the Smithsonian Institution sent professional collector, and later Yale professor, Charles Schuchert to Choctaw County to collect Basilosaurus specimens for the U. S. National Museum of Natural History. He recovered the front part of one skeleton near Melvin and the back Basilosaurus cetoidespart of another near a community known as Fail, which were combined as an almost complete composite. The Fail partial skeleton included pelvic bones and part of an upper leg, which were interesting enough to be described separately by Smithsonian curator Frederic Lucas in 1900. As an amusing aside, German paleontologist Othenio Abel was so certain that the pelvic bones of Basilosaurus cetoides belonged to a bird that in 1906 he gave the pelvis its own scientific name: Alabamaornis gigantea (a synonym of B. cetoides). The Melvin-Fail Basilosaurus skeleton has been exhibited at the Smithsonian since 1910. Some questions remain about the conformation of the hands and feet, and about the total number of vertebrae in the Melvin-Fail composite (and hence the true length of Basilosaurus cetoides in general).

The Alabama species Basilosaurus cetoides has never been found outside North America, but the slightly smaller, closely related species Basilosaurus isis is known from many skeletons in Egypt. These have pelves, and reduced, but well-formed legs, feet, and toes as vestiges of a land-mammal ancestry lost in later whales. The Basilosaurus cetoides skeletons of Alabama all come from layers of rock that were formed at the bottom of a shallow Gulf Coast sea during the late Eocene. Smaller archaic whales such as Zygorhiza kochii and Cynthiacetus maxwelli inhabited this sea as well, but Basilosaurus cetoides was the largest and most spectacular. Rocks of the period that contain fossils of Basilosaurus cetoides belong to the Jackson Group, principally the Pachuta Marl and Shubuta Clay formations Basilosaurus cetoidesin the western part of Alabama, extending into Mississippi, and the Ocala Limestone formation farther east in Alabama, extending all the way to Florida. These formations form a band of outcrops that parallel the present Gulf Coast. Stratigraphic and paleontological evidence indicates that southern Alabama was coastal during the late Eocene, and accumulation of many Basilosaurus skeletons can be attributed to a favorable living environment and to the slow accumulation of sediment that covered the animals when they died, thus enhancing fossil formation.

Basilosaurus was a specialized type of animal that did not give rise to any later whales. Abrupt global cooling of the Earth's climate at the end of the Eocene coincided with changing ocean circulation. This led to the extinction of Basilosaurus and most archaic whales around 34 million years ago. At the same time, new currents and deep ocean upwelling favored the diversification of modern toothed and baleen whales.

Additional Resources

Code of Alabama. Designation of Basilosaurus cetoides as the State Fossil of Alabama, Alabama State Legislature, 1984.